Tetralogy of Fallot and Hypoplastic Left Heart Syndrome - Complex Clinical Phenotypes Meet Complex Genetic Networks.

In many cases congenital heart disease (CHD) is represented by a complex phenotype and an array of several functional and morphological cardiac disorders. These malformations will be briefly summarized in the first part focusing on two severe CHD phenotypes, hypoplastic left heart syndrome (HLHS) and tetralogy of Fallot (TOF). In most cases of CHD the genetic origin remains largely unknown, though the complexity of the clinical picture strongly argues against a dysregulation which can be attributed to a single candidate gene but rather suggests a multifaceted polygenetic origin with elaborate interactions. Consistent with this idea, genome-wide approaches using whole exome sequencing, comparative sequence analysis of multiplex families to identify de novo mutations and global technologies to identify single nucleotide polymorphisms, copy number variants, dysregulation of the transcriptome and epigenetic variations have been conducted to obtain information about genetic alterations and potential predispositions possibly linked to the occurrence of a CHD phenotype. In the second part of this review we will summarize and discuss the available literature on identified genetic alterations linked to TOF and HLHS.

Elucidation of penetrance variability of a ZIC3 mutation in a family with complex heart defects and functional analysis of ZIC3 mutations in the first zinc finger domain.

We studied a series of 42 cases of transposition of the great arteries (TGA), a complex heart defect (CHD) that is two times more prevalent in males than in females. A mutation in the X chromosome at the ZIC3 gene was found in two affected siblings (one male, one female) and their unaffected mother. A second factor, skewed X-inactivation pattern explained the discrepancy between the daughter/mother phenotype. In this family, the missense mutation (p.W255G) was found in the first zinc finger of ZIC3, a domain that is relatively specific to each of the five human ZIC genes. It was tested further along with two other mutations of this domain (p.C253S and p.H286R). In transfected 3T3 cells, mutants p.W255G and p.H286R expressed lower protein levels, and an increased protein degradation (p.W255G only). Moreover, mutants p.C253S and p.W255G had a decreased transcription activation of the TK-luciferase reporter gene. Nuclear translocation of the three ZIC3 mutants varied considerably depending on the experimental models. Finally, p.W255G and p.H286R showed diminished activities for both left-right axis disturbance and neural crest induction in Xenopus embryos. These results suggest that mutations in the first zinc finger of ZIC3 mildly affect several functions of the protein.

A novel de novo TBX5 mutation in a patient with Holt-Oram syndrome leading to a dramatically reduced biological function.

BACKGROUND: The Holt-Oram syndrome (HOS) is an autosomal dominant disorder affecting 1/100.000 live births. It is defined by upper limb anomalies and congenital heart defects with variable severity. We describe a dramatic phenotype of a male, 15-month-old patient being investigated for strict diagnostic criteria of HOS. METHODS AND RESULTS: Genetic analysis revealed a so far unpublished TBX5 mutation, which occurs de novo in the patient with healthy parents. TBX5 belongs to the large family of T-box transcription factors playing major roles in morphogenesis and cell-type specification. The mutation located in the DNA-binding domain at position 920 (C→A) leads to an amino acid change at position 85 (proline → threonine). Three-dimensional analysis of the protein structure predicted a cis to trans change in the respective peptide bond, thereby probably provoking major conformational and functional alterations of the protein. The p.Pro85Thr mutation showed a dramatically reduced activation (97%) of the NPPA promoter in luciferase assays and failed to induce NPPA expression in HEK 293 cells compared to wild-type TBX5 protein. The mutation did not interfere with the nuclear localization of the protein. CONCLUSION: These results suggest that the dramatic functional alteration of the p.Pro85Thr mutation leads to the distinctive phenotype of the patient.

Identification, genomic organization, chromosomal mapping and mutation analysis of the human INV gene, the ortholog of a murine gene implicated in left-right axis development and biliary atresia.

Determination of left-right axis is a precocious embryonic event, and all phenotypic anomalies resulting from disruption of the normal lateralization process are collectively referred to as the lateralization defect. A transgenic mouse with lateralization defect and hepatic, kidney, and pancreatic anomalies has resulted from disruption of the inv gene by insertion of a transgene. The human ortholog is thus a good candidate for lateralization defect in humans, in particular in cases with associated hepatic anomalies. Here, we have identified, mapped, and characterized the INV human gene and screened a series of heterotaxic patients (with or without biliary anomalies) for mutation in this gene. In a German family of Turkish origin, we have found that all available affected and unaffected individuals are heterozygous for a mutation in the splicing donor site of intron 12 in the INV gene resulting in two different aberrant splicing isoforms. This can be explained either by a randomization of lateralization defects or, as suggested earlier, di- or trigenic inheritance, although we have been unable to detect, in this family, a mutation in genes known to be involved in the human lateralization defect ( LEFTY1, LEFTY2, ACVR2B, NODAL, ZIC3, and CFC1). In contrast to the mouse, the affected individuals have no biliary anomalies, and the absence of mutation in a series of seven cases with lateralization defect and biliary anomalies demonstrates that INV is not frequently involved in such a phenotype in humans.